To help illustrate a question a friend and myself were discussing this afternoon, here is a link to a video that started the thought.

http://www.youtube.com/watch?v=XgpX-ZeXgrM&feature=related

You can see the valve motion on the first set of drivers has piston valves and direct motion, evidenced by the position of the radius rod and eccentric in relation to the main driver. The second set also has piston valves, but with indirect. I'm not familiar with the engine enough to know if it is a Mallet, but if it is, was the second set of cylinders converted to piston valves?

Another question I wanted to know about after reading some of the I.C.S book on Walscheart Valve Setting is why some Mallets retained a slide valve for the second driver set? It seems to me the maintenance would be higher with a slide valve. Why did the Army prefer indirect motion on their steam locomotives? The White Pass 190 class have indirect which were Army Corps. commissioned, as well as the few 600 class Consolidations (610 at TVRM).